Add SFTP atomic upload functionality

When uploading a file to a remote directory, the file is incomplete until finished. If pushed to a directory with a webserver, clients could download partial files and be very confused (I'm sure there are other situations where this could be an issue as well). rsync doesn't have this issue -- it uploads a temporary file, then rename it once the transfer is completed. However, rsync doesn't work everywhere.

I filed an issue at ssh2, but they were of the opinion that something higher up the stack should do this sort of thing, so here we are. In particular, it would be annoying to have to reimplement the directory upload code to insert this sort of functionality -- so it would be nice if either:

• There was a hook that made this possible for a user to implement
• Or there was an option that could be passed in that would activate the 'atomicPut' functionality

I've created a monkeypatch'ed version of this, works great on my server. It pushes as a dot + filename so that the webserver doesn't show it to clients, and then changes the filename once the upload is finished. I'd rather get rid of the monkeypatch, and I'm happy to do the changes + make a PR here if you'll accept it.

No repository file or test is named. Start by locating the SFTP file and directory-upload entry points, then compare the linked monkeypatched implementation. Done means users can opt into or hook atomic uploads so transfers use a temporary name and rename only after completion.